Amplifiers are used to amplify signals to obtain desired field intensity. In a case where desired electric power cannot be obtained with only one amplifying device, two or more amplifying devices are connected in parallel, and the outputs of the amplifying devices are combined in the coordinate-phase, so as to obtain desired power (see “Advanced Techniques in RF Power Amplifier Design”, by Steve Cripps, Artech House, 2002 (ISBN-13: 978-1580532822), for example).
Since amplifying devices used in amplifiers have nonlinearity, those amplifying devices have the problem of outputting signals accompanied by nonlinear distortion. To counter this problem, amplifying devices may be operated in low-output regions with high linearity. In doing so, however, another problem is caused, as the efficiency becomes much lower in the low-output regions.
To counter those problems, a push-pull technique has been suggested. By the push-pull technique, nonlinear amplifying devices are connected in parallel, so as to operate amplifiers with high efficiency. Linear outputs are then obtained by varying the periods of time for operating the respective amplifying devices. Alternatively, it is possible to employ a technique of using a linearizer circuit for erasing distortion that is output from nonlinear amplifying devices. By a regular technique of using a linearizer circuit, signals having the opposite distortion of that of amplifying devices are provided in advance, and signals formed by adding the signals having the opposite distortion to input signals are supplied to the amplifiers. This circuit is called a predistortion-type linearizer circuit.
There is also a feedforward-type linearizer circuit that extracts distorted signals from output signals of amplifying devices, generates the reverse-distorted signals based on the extracted distorted signals, and subtracts the reverse-distorted signals from the output signals of the amplifying devices, so as to eliminate the distortion (See “Advanced Techniques in RF Power Amplifier Design”, by Steve Cripps, Artech House, 2002 (ISBN-13: 978-1580532822), for example). There is also a technique of dividing signals on a frequency axis with the use of two different bandpass filters (see JP-A 2002-513227(KOKAI), for example).
By any of the above techniques, intermodulation distortion existing close to the signal band can be reduced, but harmonic distortion cannot be eliminated. Therefore, conventionally, a low-pass filter is added in a stage behind the amplifier, so as to eliminate harmonic distortion. However, the addition of a low-pass filter causes the problem of an increase in power loss.